A SCUBA/Spitzer investigation of the far-infrared extragalactic background

We have measured the contribution of submillimetre and mid-infrared sources to the extragalactic background radiation at 70 and 160 mu m. Specifically, we have stacked flux in 70- and 160-mu m Spitzer Space Telescope (Spitzer) observations of the Canada-United Kingdom Deep Submillimetre Survey 14-h field at the positions of 850-mu m sources detected by SCUBA and also 8- and 24-mu m sources detected by Spitzer. We find that per source, the SCUBA galaxies are the strongest and the 8-mu m sources the weakest contributors to the background flux at both 70 and 160 mu m. Our estimate of the contribution of the SCUBA sources is higher than previous estimates. However, expressed as a total contribution, the full 8-mu m source catalogue accounts for twice the total 24-mu m source contribution and similar to 10 times the total SCUBA source contribution. The 8-mu m sources account for the majority of the background radiation at 160 mu m with a flux of 0.87 +/- 0.16 MJy sr(-1) and at least a third at 70 mu m with a flux of 0.103 +/- 0.019 MJy sr(-1). These measurements are consistent with current lower limits on the background at 70 and 160 mu m. Finally, we have investigated the 70- and 160-mu m emission from the 8- and 24-mu m sources as a function of redshift. We find that the average 70-mu m flux per 24-mu m source and the average 160-mu m flux per 8- and 24-mu m source is constant over all redshifts, up to z similar to 4. In contrast, the low-redshift half (z < 1) of the of 8-mu m sample contributes approximately four times the total 70-mu m flux of the high-redshift half. These trends can be explained by a single non-evolving SED.